Synthesis, molecular structure and optical spectral studies of syn-dithia benzothiazolophane and anti-bis-lactone benzothiazolophane

Synthesis of syn-dithia-benzothiazolophane 1 and anti-bis-lactone benzothiazolophane 2 has been carried out by well established protocols as outlined in Schemes 1 and 2, respectively. The 1H NMR spectra of heterophanes 1 and 2 were found to be temperature independent in the range of255 to þ120 8C which indicates conformationally mobile nature of these molecules. In comparison to plain CHCl3, the UV–visible spectra of 1 and 2 measured in CHCl3 containing trifluoroacetic acid (TFAA) exhibited bathochromic shifts by 25 and 19 nm, respectively. The red shifts are attributed to the increased charge transfer interaction arising from the donor phenyl ring to the acceptor benzothiazolium ring. The emission spectra of thia-bridged 1, both in the neutral as well as the protonated forms are accompanied by large Stokes shifts (Dn ¼ 127–156 nm) with the emissions most likely originating from the locally excited states. The bis-lactone 2 in neutral CHCl3 solvent also exhibited a single locally excited emission, but with a relatively lower Stokes shift (Dn ¼ 69 nm). Interestingly, unlike 1, bis-lactone 2 in the protonated form displayed both local as well as what appears to be an intramolecular charge transfer emission. The emission behavior of 1 and 2 has been tentatively rationalized on the basis of HOMO–LUMO interaction. Molecular modelling of 1 and 2 generated five reasonable conformations within 5 kcal/mol depending upon the orientation of the benzothiazole rings and the heteroatoms in the connecting bridges. The transannular distances in 1 and 2 were found to be ca. 2.8 and 3.6 A ° , respectively. While the rings in 2 are nearly planar, however, in the case of 1 the stacks are slightly curved signifying molecular strain in the letter system. Based on dynamic 1H NMR spectral analysis and molecular modelling, we propose that these molecules exist as a rapidly equilibrating mixtures of conformers. q 2004 Published by Elsevier B.V.